Liquid circulating heating system



y 22, 1952 L. H. GILLICK ET AL LIQUID CIRCULATING HEATING SYSTEM 5 Sheets-Sheet 1 Filed Aug. 4, 1949 M66 W61 30m 6& M

T J fl y 22, 1952 L. H. GILLICK ET AL 2,604,269

LIQUID CIRCULATING HEATING SYSTEM Filed Aug. 4, 1949 3 Sheets-Sheet 2 INVENTORSQZ .Z a". 17 fill y 22, 1952 L. H. GILLICK ET AL 2, 04,269

LIQUID CIRCULATING HEATING SYSTEM Filed Aug. 4, 1949 3 Sheets-Sheet 5 1/ V ff P/mm/ey III 1 76 o F VAL v5 540M156 Kg Patented July 22, 1952 LIQUID CIRCULATING HEATING SYSTEM Laurance H. Gillick, Evanston,

Timothy J. Le-

hane, North Riverside, and Norman 0. Kirkby, Elmhurst, Ill., assignors to Vapor Heating Corporation, Chicago, I11.

, a corporation of Dela- Application August 4, 1949, Serial No. 108,574

11 Claims. 1

This invention relates to space heating systems and more particularly to heating systemsof the above class comprising a primary heating apparatus and a secondary apparatus which cooperate, under predetermined conditions, to maintain the said space at a desired temperature.

One of the objects of the invention is to provide a heating system of the above general class in which the secondary heating apparatus includes a plurality of heat radiating units, each arranged for heating a particular zone of an enclosed space and each unit being independently thermostatically controlled in relation to the heating requirements of its associated zone.

Another object of the invention is to provide improved arrangements and constructions whereby a heated liquid is circulated by forced circulation through the secondary heating apparatus under certain operative conditions and during other conditions is circulated through said heating apparatus by gravity.

A further object of the invention is to provide, in a heating system utilizing relatively long heat radiating units, novel means for equalizing the temperature of heat radiated from the full length of said units and thereby avoid extreme differentials in temperature between the inlet and outlet end portions thereof.

Other objects and advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings in which:

Fig. 1 is a diagrammatic plan view of a heating system in accordance with the invention, shown as applied to a railroad passenger car.

Fig. 2 is a longitudinal sectional view taken substantially on line 2--2 of Fig. 1.

Fig. 3 is a sectional view illustrating a spring opened solenoid valve for controlling the admission of heating medium to one of the smaller length radiators shown in Fig; 1.

Fig. 4 is adiagrammatic view, partly in section, of a by-pass valve associated with the liquid circulating system.

Fig. 5 is aside view, partially in section, illustrating a connection of the return end of a radiator connected with the inlet end thereof.-

Fig. 6 is a diagrammatic view of a steam supply valve, hereafter referred to as a vapor regulator, which so regulates the delivery of steam to heat exchangers of the primary and secondary heating apparatus as to maintain the steam vapor therein at substantially atmospheric pressure.

Fig. 7 is a wiring diagram of the electrical circults for controlling the operations of the several 2 elements of the primary and secondary heating apparatus. I

Referring to the drawings, the invention, for purposes of illustration, is shown as applied to a railroad passenger car, indicated diagrammatically in Figs. 1 and 2. It is to be understood, however, that the invention is not to be limited in its use to the particular application described, but may be used to heat any enclosure where steam is available as a primary source of heat. The heating system, as herein illustrated, comprises a primary heating apparatus which is effective to deliver heat into the interior of the car, herein exemplifying any enclosed space, whenever heat is required, and a secondary heating apparatus which is effective to supply additional heat to various zones of the enclosed space when such additional heat is required.

PRIMARY HEATING APPARATUS The primary heating apparatus includes an air heater A, located in an air duct B, and a blower C for forcing a stream of air through the heater A; the heated air being delivered into the upper portion of the enclosed space, for example a rail way car, through dischargeports D formed in the air duct B. The air heater A, in the present disclosure, utilizes steam as a heating medium. The steam flows from a source of supply, for example, steam train line E, through a vapor regulator F and a pipe G to a solenoid admission valve H interposed in the pipe G. The said admission valve H is normally closed by a spring I and is opened by the energization of the solenoid J. The solenoid, as will be hereinafter described, is controlled by temperature responsive means including a primary heat thermostat K which is responsive to the general temperature of the car as a whole. For example, when the general temperature of the car falls below the functional setting of thermostat K, the valve H is opened to admit steam into the air heater A. Condensation from the steam is discharged through a pipe L to drain pipe M of the pressure regulator F.

SECONDARY HEATING APPARATUS I General function The secondary heating apparatus cooperates in the several zones. The said liquid is heated in a heat exchanger In which receives its source of heat from the steam train line E. The steam passes from the steam train line E at relatively high pressure through a shut-off valve in pipe l2 and into the; vapor regulator F; which regulator functions to deliver the steam at substantially atmospheric pressure to the heat exchanger It. The low pressure steam passing from the vapor regulator F to the heat. exchanger l through pipe M and the condensation and surplus steam is discharged through return-pipeto the drain outlet l6.

VAPOR REGULATOR.

The vapor regulator F is illustrated diagrammatically in Fig. 6, since the: specific: structures thereof are not required for a thorough understanding of the present, invention. It will be sufiicient, therefore, to confine the description of the said regulator to its control functions.

These control functions are as follows; The

steam from thetrain line E enters chamber I5 and passes through valved port ||5 into chamber |-1-. When the steam in chamber |=1 reaches a predetermined low pressure, the pressure actuated-valve I8 is closed'to a position to maintain the-desired pressure in said chamber l1. 1 When steam is required in the heat exchanger H), the

steam passes through a valve controlled port l9 into a lowpressure chamber 20 from which it is delivered through pipe |'4 into the heat exchanger Hi. When the heat exchanger is filled with steam, any surplus passes out through'the return'pi'pe I5 to the drain outlet I5. The con- 'densation is discharged'through thed'rain outlet,

but the steamflows upwardlyinto a thermostat chamber 2| ofthevapor regulator and applies heat to a bellows form of thermostat 22. The expansion of the bellows thermostat rocks a lever 23 and thereby closes a spring opened valve 24 to closeport l9 and thereby shuts off thedelivery 'ot steamto the heat exchanger.

Inasmuch as th ou-tlet pipe |5and thermostat chamber 2| are open to the atmosphere, the steam pressure in the heat exchanger will be maintained at substantially atmospheric pressure.

' The supply of steam to the heat exchanger can also-beshut off by opening a valve 25 and thereby permit steam to pass direct from the chamber 2|} into thethermostat chamber 2|. This valve will'open, inthe' event that the pressure in chamber 20 exceeds a predetermined value, by compressing the spring' 26--which normally holds the ing heat into the-enclosed space and atleast one of the several secondary heat thermostats hereinafter mentioned is calling for. heat.

SECONDARY HEATINGAPPARATUS Zone heating The enclosed space, herein exemplified as the interior-of a railway car, is divided into several zones designed O, P. Q. R, S, T. The zones 0, P, Q: and R; are served by radiators designated,

respectively, in Figs. 1 and 2 by numerals 21, 28,

29- and 30, but the zones Sand T are served by a single radiator'3 l which is substantially longer than the others. The radiator 28 is also relatively long since it extends the full length of the passenger space, of which zone P represents one side of the car. The shorter radiators 21, 29 and 30 are utilized to heat smaller zones, but inasmuch as these zones are located atthe ends of the car and ordinarily have greater heat loss, the radiators are provided with a larger number of heat dissipating fins, per linear foot, than the radiators. 28 and 3|.

The liquid heating medium is delivered to the several radiators by means of a main circulating loop which may be located between the main and sub-flooring of-the car and comprise a supply conduit 32' and a return conduit 33. A pressure responsive valve 34 connects the remote end of supply conduit 32. with the entrance end of the return conduit 33. Consequently, the liquid heating medium mayv circulate continuously through the main loop when the pressure therein issufiicient to unseat the pressure opened valve 34.

Connected in said main loop, in series with conduits 32 and 33, is a motor-drivenpump. 35 and connected in parallel with said pump is a by-pass 36 which operates to permit the system to function by gravity circulation when the motor of pump 35 is de-energized, for example, during lay-over periods when it is. desired .to maintain'a lower temperature in the car.

Referring to Fig. 4, the by-pass includes. a chamber 31- having ports 38 and 39' connected by conduits 48 and 4|, respectively, to the return conduit 33. A spring biased valve 42. is contained in the chamber 31 and is normally retained in the position shown by the dotted lines, but under certainconditions of. operation is caused to close the port 39 as" will be hereinafter explained.

It will be observed that the inlet end. of'each radiator 21, 28, 29, 30 and-3| is provided, respectively, with separate solenoid admission valves 43, 44, 45, 45 and 41. The solenoid valve 43 for radiator 21, shown in Fig. 3, is connected by branch pipe to the flow conduit 32 of'the circulating loop and outlet end of the said radiator is connected by pipe 49 to the return conduit 33 of said circulating loop.

Considering said valve 43 as representative of the valves for the shorter radiators, it is shown in Fig. 3. It is normally held in its open position by means of a spring 50 so that the heated liquid will flow from the flow conduit 32, through pipe 48, admission valve 43, when'the pump 34 is operating. When the pump is not operating, the liquid, when it is heated, will fiow by gravity through the radiator 21 in the same direction as above described, but it will by-pass the pump 35. During-this gravity circulation, the spring operated valve 42 assumes the full line position, shown in Fig. 4, so that the liquid will flow through the by-pass from the return conduit 33 at the suction side of the pump 35 through pipe 4|, valve chamber 31, and pipe to the conduit 33 at the pressure side of the pump.

Considering next the radiator 28, which is representative of the longer radiator units, the said unit 28 is provided with a fitting, indicated genmany by the numeral 5|, cooperating with said solenoid valve 44 and is connected to the flow conduit 32 of the circulating loop by conduit 53. The outlet end of the radiator unit 28 is connected to the return conduit 33 of said circulating loop by conduit 54.

The fitting 5| and solenoid valve 44 areshown diagrammatically in detail in Fig.5. Thesaid fitting includes a housing in which is contained a nozzle 52 having a narrowing passageway, the discharge end of the nozzle being directed into the upper run of the radiator unit 28 and projecting beyond the mouth of a port opening 55 provided in the wall of the housing. A conduit 56 connects the port opening 55with the outlet end of the radiator 28 which also connects into the return conduit 54.

The function of the fitting 5| is to equalize the temperature of the heating medium flowing through the relatively long radiator unit, so as to avoid extreme differentials in temperature between the inlet and outlet end portions-of the radiator. It is known that in radiators com posed of long conduits, gradations in temperature decrease progressively throughout the length of the conduit. This condition is remedied by the use of the fitting 5! which functions in the manner of an injector, as will be presently explained.

As the heating medium is caused to fiow through the narrowing passageway of the nozzle 52 into the inlet end of the radiator 28, its velocity of flow entrains from the connecting conduit 53, a portion of the liquid from the discharge end of the radiator and recirculates the entrained liquid through the radiator. Thus,

the heating medium entering the radiating unit 1 at a relatively higher temperature is intermixed with the spent medium entrained from the dis charge end of the radiator, thereby reducing the temperature of the heating medium introduced,

but by recirculating a portion of the spent medium maintains greater uniformity in heat radiation throughout the full length of the radia tor and therefore increases its heating efficiency as a whole.

The radiator units 29 and 36 are connected in the supply and return conduits 32 and 33 of the circulating loop in substantially the same manner as described in connection with radiator 21; the inlet and outlet of radiator 29 being connected into the loop by branches 51, 58 and radiator 30 being connected into said loop by branches 59 and Bil. The long radiator}! is similar to the radiator 28 and is provided with a fitting 5i which corresponds in structure to fitting 5i and is connected into the circulating loop by branch conduits 6i and 52. Each conduit leading from the'fiow' conduit 32 of the circulating loop to a radiator admission valve .is provided with an orifice fitting 63. The orifice passages of the said fittings are so proportioned, having regard to the friction-location and heating requirements of the several radiators, to balance the distribution of heating medium among the several radiators. .Also all admission valves are normally open as described in connection with valve 43, whereby it is necessary to energize their associated solenoids I24, 524 I2 EN and I2, to close said valves.

The primary heating apparatus, as before indicated, is responsive of the car or space being heated. The secondary heating apparatus delivers heat into the several zones of the car, but it is, dependent for its operation uponthe condition of the controls of the primary heatingapparatus, since the primary apparatus must be in operating condition before any part of the secondary apparatus.

The operation of the improved heating system will best be understood by reference to Fig. 7 of the drawings which show the electrical wiring diagram thereof.

to the general temperature CONTROL CIRCUITS Primary apparatus Considering first, the electrical circuits for opening the steam valve H to deliver steam to the air heater A of the primary apparatus: These circuits are closed through a main relay 64 under the control of said primary heat thermostat K. The said thermostat is provided withan electrical auxiliary heater 65 which, for the purpose of establishing its functional setting, receives electrical heating current through a circuit leading from positive line 66 through wire 61, variable resistor 68, fixed resistor 69, wire 10, auxiliary heater 65, and wire II to the negative line T2. The setting of the variable resistor 83, shown in Fig. 7, may be regarded, for purpose 1 illustration, as establishing the functional setting of thermostat K at 76 F. This temperature setting may be raised or lowered by appropriate adjustment of the variable resistor 68.

When the thermostat K calls for heat, the relay 64 is energized to close, through its contact 80, a circuit for energizing the solenoid J of steam valve H and thereby deliver steam to the air heater A. The circuit for energizing said relay 64 leads from positive line 66 through wire 73, resistor 14, relay solenoid 15, wire 16, resistor Ti and wire 18 to the negative line 12. The closing of said. relay establishes the said energizing circuit for the steam valve solenoid J, which circuit is as follows: Positive line 66, wire 19, closed relay contact 81!, wire 8|, solenoid J of steam valve Hand wire 82 to negative line 12.

Simultaneously with the energization of the said circuit for opening the steam valve H, a circuit is closed through energized contact 83 of relay 64 to establish a circuit for applying additional heating current to auxiliary heater 65.- This additional circuit leads from positive line 66 through wire 84, relay contact 83, cycle're- 'sistor'85, having a heat control value of 2 F.,

and wires 86 and Hi through the heater 65 and said wire ii to the negative line 12. The 2 of heat added to the thermostat K as a result of the last mentioned circuit reduces the temperature setting of thermostat K to 74 F. until its mercury column 3! engages its contact 12, This functioning of the thermostat K results in deenergizing the relay 64, since, under such conditions, the electric current is by-passed around the relay solenoid 75. This by-passing circuit leads from the positive side of the solenoid 15 through wire 88, contact 89 of thermostat K and through its mercury column 81 to wire 90 and wire 16, resistor 17 and'wire 18 to the negative line 12. The said deenergization of relay 64 opens relay contact 83 and therefore removes the 2 of heat from the auxiliary heater B5 of thermostat K. Theauxiliary heater therefore cools and CONTROL CIRCUITS Secondary apparatus lieferringnow to the circuits for controlling the operations of the several radiators of the secondary heating, apparatus: 'I'he radiators 27, '28 and 29 located in zones 0, P and Q at one side of the car, are controll'edby.theirindividual thermostats BI, 92 and 93, provided with electrical heaters 94, 95 and '96-; The radiators 30 and 3| are controlled by thermostats 9'I"and98provided withelectrical heaters and I00. Theseveral thermostats just described are secondary heat thermostats and are set :to function at the. same temperature asithe primary 'heatithermostat K, for example 76 F.', and the auxiliary heatersof the-said thermostats have'ithe same heat'control value (2 'F.) as the electrical heater-65 of the primary heat thermostat K.

Dependence of secondary apparatus Thesecondaryheat thermostats 9i, 92,93, 9] and 98 may functionto shut off the delivery'of steam to'their'respective radiators, but heating medium willbe delivered' to said radiatorsonly when the' primary heat thermostat K1 is calling for heat; In other words, the secondaryapparatusiswdepend'ent upon the conditionrof the controls of' the primary--"apparatus; 'IZhisx-dependence is obtained by utilizing the conjoint functionsofthe primary heat thermostat'K land thesecondary heat'thermostatsSI, 92, 93, 91 and 98- to control thedeliveryof steam to the liquid heater I0. This result isobtained by controlling the energization of" solenoid N of spring closed valve 25 of the vapor regulator F, Consequently, the energizing circuitfor the said solenoid N is connected in series through a-group'of relays I I, I02, I03, I04 and-I05; the'said relays being connected in parallel throughthe main relay 64, the latter of which is controlledby primary heat thermostat K. Consequently, when the primary heat thermostat K is calling for heat, the solenoid N' of valve 25 will be deenergized to shut off the direct passage'of'steam from chamber-20 to the thermal chamber 2I of the vapor regulator F and'thereby permit the steam to pass from chamber 20 of thevapor regulator into the heat exchanger I0, provided, of course, that at least one of the secondary thermostats is calling for heat.

The energizingcircuit. for said solenoid 'N for opening valve 25 leads from the positive line 66 through conductor I06 which is connected" in series through the top contacts I01, I08, I09, H0 and III, of the said relays IOI, I02, I03,.I04 and I05, respectively, and thence through the solenoid N and wire II2 to the'negative line. The actuating solenoids of thelast mentioned group of'relays are energized by circuits closed bythe energized relay 64. Considering first the circuit for energizing the actuating solenoid of relay WI: The circuit leads from positive line 66 through bus connector II3, energized closed contact II4 of relay 64, wire H5; resistor IIS', wire I", solenoidcoi-l II8 of relay I M, wire II9, resistor'I20 andwireIZI tothenegativeline. The energization of said relay solenoid H8, in addition to being controlled .by the energization of the main relay 64, is also. controlled; by the functioning of the-secondary heat. thermostats! to by-pass'the electric current around solenoid IIO. This by-pass connection includes wire. I22 leading from wire" I H, at the positive-side of solenoid II8, to the upper contact of thermostat 9| and wire I23 which leads from the lower contact of thermostat ill to the wire H9 at the negative side of solenoid II8. Consequently, when the temperature of the zone 0, served by radiator 21, is sufllcient to cause secondary heat thermostat 9I to function, the electric current is bypassed around solenoid II8 ofirelay IN and permirsl-this relay'*'to;close, as is indicated-in the drawing, andtherefore energizes the solenoid vI 24 of- .thel-radiator valve :43 I to close 1 the said, valve against the compression of spring 50. The valve 43 will be held closed until the temperatureof zone 0 is loweredby; loss of heat therefromor until .the'electric heater 94 isdeenergized to remove:- the electrical heat (2) fromthe thermostat.

The secondary heat thermostat 9I will function. when the temperature in zone 0, in which the thermostat is located, reaches the adjusted functional setting of the thermostat, for example.'74- R, but'the thermostat heater94 will retain itsheat until the. primary heat thermostat Kv reachesitsadjusted functional setting of 74 F. This: condition is-dueto the fact that the heater 94 15: connected through contact I25, relay 64, cycleresistor I21, wire .128, heater 94, wires I29 and I2Ito: the negative line 12. Consequently the secondary: heat thermostat9I will cycle,- by removal and re-application of auxiliary heat thereto, during the cycling operation of th primaryheat thermostat K. Under such conditions, the secondary heat thermostat 9I will cycle to deliver. small quantities of' heating medium to radiator 21 until the temperature of said. zone 0 reaches the normal setting (76 F.) of the said thermostat 9 I.

The radiatorsrn, 29', 30 and 3|, as before indicated, .are controlled in the same manner as said radiator2l, by means of the secondary heat thermostats 92', 93, 91 and 98, through their associatedrelays I02, I03, I04 and I05. Inasmuch as the energizing circuits for the solenoids H8, IIBP, 8, and II8 for the relays I02, I03, I04 and I05, respectively, are energized through contacts H4, H4, I'I4, and II4 of relay 84 in the same manner and simultaneously with the solenoid II4 of relay IOI, these several circuits are identified'by the same numerals as the energizing circuit of solenoid II8 with the addition of exponents a, b, c and d. The energizing circuits for the solenoids I24, I24, I24, and I24 for the admission valves 44, 45, 46 and 41 forthe radiators 28, 29, 30 and 3I and the by-passing circuits for deenergizing said solenoids ar substantially the same as corresponding portions of the circuits associated with relay WI and secondaryheat thermostat 9|. Therefore in order to avoid repetition of description th several circuits are identified by the same reference numerals as the corresponding circuit associated with relay IOI and thermostat 9 I, with the addition of the exponents a, b, c and d.

The pump 35 is controlled by a relay I30 and isoperative on an independent circuit. The relay I30 isnormally energized to maintain the contact bar I3I out of engagement with thecontact points I32 and I33. When there is a requirement for" heat in any one of the zones, as was hereinbefore explained, any of the relays such as IOI, I02, I03, I04 or I05 in becoming energized, will effect separation from the contacts'from' the respective contact bars to open the circuit through the various relays to deenergize the relay I30 which causes the contact bar I3I to engage contacts I32 and I33 to effect operation of th pump 35. The blower. C is connected to the line 56 and I2 through conductors I34 and I35 through switch I36.

The operation of the by-pass 36 will be understoodiby referring to Fig. 4. As heating medium is pumped by the pump 35 in the direction of the arrows, the pressure createdin the conduits acoaaee 33 and 40 causesthe valve flap 42 to seat against the port opening of pipe 4| 'tojprevent recircu-v lation of the medium through conduit 4|. However, when the car is out of service'for any extended periodof time and it is desired to'maintain heat ,in-the car without burdening the battery system thereof, the valve flap 42 is' caused by its spring to open, permitting heating medium to flow "by gravity through conduit 4] through the chamber 31, through th conduit 40 into conduit33 thereby by-passing the pump 35 which would normally prevent such movement of heating medium.

7 We claimi '1. A heating system comprising a primary heating apparatus and a secondary-heating apparatus, each including a heat radiating unit and an electrical valve for controlling the admission of heating medium thereto, means including a primary heat thermostat and a secondary heat thermostat and separate relays associated therewith for controlling the operation of said valves, the relay for controlling the secondary apparatus being energized by a circuit connected through the relay associated with the primary heat thermostat and the secondary thermostat being interposed in a by-pass circuit connected inthe energizing circuit of its associated relay, whereby the functioning of either thermostat while theother is calling for heat is effective to deenergize the relay associated with said secondary heat thermostat.

2. A heating system comprising a primary heating apparatus including a heat radiating unit and a solenoid valve for controlling the admission of heating medium thereto, a secondary heating apparatus including a heat radiating unit delivery of heating medium theerto, means in and a solenoid valve for controlling the delivery of heating medium thereto, means including a primary of heating medium thereto, means including a heat thermostat and a relay switch associated therewith for controlling the energization of the solenoid valve associated with the primary heating apparatus, means including a secondary heat thermostat and a relay switch associated therewith and electrically connected through the first mentioned relay switch, whereby the secondary heating apparatus can be effective only when the primary apparatus is operative and the secondary heat thermostat assoeiated with the last mentioned relay is calling for heat.

3. A heating system comprising a primary heating apparatus and secondary heating apparatus, each including a heat radiating unit in which a heating medium is adapted to circulate, a heat exchanger having its source of heat derived from a steam line and adapted for heating a liquid medium adapted for circulating in said secondary heating apparatus, means including a primary heat thermostat associated with said primary heating apparatus for controlling the flow of heating medium thereto in response to the action of said primary heat thermostat, and means associated with said secondary heating apparatus and with said first mentioned means and adapted to cause the flow of heating medium through said secondary heating apparatus only when said primary heating apparatus is operative.

4. A heating system comprising a primary heating apparatus including a radiating unit in which steam is used as a heating medium, and a secondary heating apparatus including a radiating in which a heated liquid is adapted to circulate, a primary heat thermostat associated with th'e'primary heating apparatus, a secondaryheat thermostat associated with the-secondary heating apparatus, means associated with v said primary heating apparatus adapted to cause the flow of steam therethrough in response to the action of its associated thermostat, means associated withsaidsecondary heating apparatus and with said firstmentioned means and adapted to cause the flow of heating medium through said secondary heating apparatus only when said primaryheating apparatus is operative. l 5. A heating system comprising a.- primary heating apparatus including a .heat. radiating unit in which a heating medium is adapted to circulate, a primary heat thermostat associated with said primary heating apparatus, all relay switch in electrical connection with said primary heat thermostat, a solenoid valve associated with saidradiating unit for controlling the flow of said heating .medium therethrough, said solenoid valve. being in electrical connection withsaid relay, a heat exchanger adapted for heating 'a liquidmedium, a' conduit loop connected to said heat exchanger, pump, means for'circulatingsa'id medium through saidconduit loop, a plurality of secondary heating elements each of which'is connected into said loop, a separate secondary heat thermostat associated with'each of said secondary heating elements, a separate relay switch in electrical connection with each of said secondary heat thermostats and said first mentioned relay switch,'a solenoid valve associated with each of said heating elements, each of said solenoid valves being in electrical contact with corresponding relays, whereby each of said secondary heating elements are independently operable but only when said primary apparatus is in operation.

6. A heating system comprising a primary heating apparatus including a heat radiating unit in which a heating medium is adapted to circulate, a thermostat associated with said primary apparatus, a relay switch in electrical connection with said thermostat, a solenoid valve associated with said radiating unit for controlling the flow of said heating medium therethrough, said solenoid valve being in electrical connection With said relay, a heat exchanger adapted for heating a liquid medium, means for supplying steam thereto, a secondary heating apparatus including a radiating unit connected to said heat exchanger, pump means for circulating heating medium through said radiating unit, a thermostat associated with said secondary apparatus, a relay switch in electrical connection with said thermostat associated with the secondary apparatus and with said first mentioned relay switch, a solenoid valve associated with said radiating unit for controlling the flow of said heating medium therethrough, said solenoid valve being in electrical connection with said relay, means including a solenoid valve effective when energized to shut on" the delivery of steam to said heat exchanger, and means defining an energizing circuit for the last mentioned solenoid valve connected through the relay associated with the secondary heating apparatuses, whereby steam is delivered to said heat exchanger only when both said thermostats are calling for heat.

7. A heating system as defined in claim 5 having means for by-passing heating medium around the pump means so as to permit circulation of said medium by gravity.

i i '8 A heating" system as defined in claim 5" in which the heating medium in the primary heatirig apparatus is steam, and which includes thermostatically controlled means for regulating the pres's'ure of the steam entering intothe heat exchanger;

9. A heating systenr as defined in claim 5 havingmeans associated with a radiating, unit of the secondary heating appartus for recirculating iii-intermixed relation a portion of spent heating medium leaving said unit with fresh heating medium entering into said unit to equalizethe temperature throughout the full length of said unit.

101A heating system according to claim 6' in which the secondary heating apparatus includes a; plurality of separate radiating units, a solenoid admission valve for each unit, separate control thermostats and solenoid switches associated therewith, and in which the solenoid valve for controlling the delivery of steam to the said heat exchanger is connected in series through each of the relay switches associated with the several radiating units of the secondary apparatus,

whereby steam is supplied to the heat exchanger when the primary heating apparatus and any one of the radiating units of the secondary apparatus is effective.

11". A heating system according to claim 6 in which the secondary heating apparatus includes a plurality of separate radiating units, a solenoid admission valve for each unit, separate control thermostats and solenoid switches associated therewith and connected in parallel througl-r-the solenoid switch associated with the primary heating apparatus, and in which the solenoid valve for controlling thedelivery of steam to the said heat exchanger is connected in series through each of the relay switches associated with the several radiating units of the secondary apparatus, whereby steam is supplied to the heat exchanger when the primary heating apparatus and any one of the radiating units of the" secondary apparatus is effective. I

LAURANCE H. GILLICK.

TIMOTHY J.

NORMAN O; KIRKBY'.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,196,687 Steinfeld Apr; 9, 1940 2,240,731 Van Vulpin May 6, 1941 2,355,040 Alexander et a1. Aug. 8) 1944 2,489,652 Lehane' Nov. 29, 1949 

